1. Field of the Invention
The present invention relates to a thermal recording printer, and more particularly to a thermally printer having a print head or recording head which can reciprocate to print data.
2. Description of the Related Art
A thermal printer which ink is thermally transferred onto the paper is a non-impact printer. It is advantageous in some respects. First, it makes little noise while printing data. Second, it can print clear characters even on ordinary paper. Third, it is easy to maintain good printing conditions. For these advantageous features, the thermal printer is used in great numbers in office automation apparatuses of various types.
Ink ribbon shown in FIG. 1 is used for a thermal ink-transfer printer. As is shown in FIG. 1, ink ribbon 103 consists of a strip of base film 101 and an ink layer 102 coated on the base film 101. In use, the ink ribbon 103 is set in contact with the print head 105 on one side, and with recording paper 106 on the other side, as is illustrated in FIG. 1. More precisely, the ribbon 103 contacts the heat-generating elements 104 of the print head 105. The print head 105 prints data, such as characters and images, on recording paper 106, in the following way.
First, an electric current is supplied to those of the heat-generating elements 104 which have been selected in accordance with the data signals supplied to the print head 105. The selected heat-generating elements 104 generate Joule heat. The Joule heat makes those portions of the ink layer 102 which face away from the selected heat-generating elements 104 soften and, thus melted ink finally forms. The melted ink stick onto the recording paper 106 and is transferred onto it. Thus, the print head 105 prints the data, which is represented by the data signals, on the recording paper 106. During this data-printing process, the ink ribbon 103 is moved relative to the print head 105 at speed V.sub.R in the direction of arrow A, and the head 105 is moved relative to the ink ribbon 103 at speed V.sub.H in the direction of arrow B.
To increase the speed at which such the thermal ink-transfer printer can print data, the print head 105 must print data not only while moving in the direction of arrow B, but also while returning to its home position in the direction of arrow A. In other words, the head 105 must print data while reciprocating over the recording paper 106. It is regarded as difficult to make the head 105 achieve such a bidirectional printing. Why it is so regarded will be explained.
The print head 105 transfers each of the ink dots from the ink ribbon 103 to the recording paper 106 in the following manner. When one of the elements 104 of the print head 105 generates heat, that portion of ink layer 107 which faces away from this heat-generating element is first softened and then melted. The melted portion of the ink layer 107 is not immediately transferred onto the recording paper 106. Rather, it is when the melted portion cools and becomes sufficiently adhered to the paper 106 that it is peeled off the ink ribbon 103 and transferred to the recording paper 106. This portion of the ink layer 102, being transferred onto the paper 106, is shown at numeral 108 in FIG. 1.
Obviously, some time elapses from the moment said portion of the ink layer 102 is heated to the moment it is transferred onto the recording paper 106. Hence, the heat-generating element 104 has been moved for some distance in the direction of arrow B when the portion of the ink layer 102 is transferred from the base film 101 to the paper 106. The portion 108 of the ink layer 102 must be kept in contact with the recording paper 106 all the time it is moved in the direction of arrow A for some distance. This distance is somewhere between several millimeters and a few centimeters, in accordance with the printing speed of the thermal ink-transfer printer.
In order to keep the portion 108 of the ink layer 102 in contact with the paper 106 while the portion 108 is moving in the direction of arrow A for said specific distance, a ribbon-holding bar 109 is used. This bar 109 is spaced away from the head 105 for said distance and moves together with the head 105 at the same speed in the same direction. Once the ink ribbon 103 has passed the ribbon-holding bar 109, it is guided in a path extending at a predetermined angle to the recording paper. As that portion of the base film 101, on which the portion 108 of the ink layer 102 is coated, passes the ribbon-holding bar 109, the portion 108 is gradually peeled off the film 101 and transferred onto the recording paper 106, from its front edge to its last edge.
That portion of the ink ribbon 103, which extends between the heat-generating element 104 and the ribbon-holding bar 109, is held in contact with the recording paper 106. This portion of the ribbon 103 does not move relative to the paper 106 at all. Therefore, the print head 105 must move at the same speed as the ink ribbon 103, but in the opposite direction. In other words, the head 105 must move relative to the paper 106 in one direction (arrow A) at the speed V.sub.H, and the paper 106 in one direction (arrow A) at the speed V.sub.H, and the ribbon 103 must move relative to the head 105 in the opposite direction (arrow A) at the speed V.sub.R ; V.sub.H =-V.sub.R.
As may be understood from the above, the electrothermal printer is characterized in that following respects:
(1) The softened and melted portions of the ink layer are transferred from the ink ribbon onto the recording paper, while the relative speed between the ink ribbon and the recording paper is maintained at zero.
(2) Once any length of the ink ribbon has been used, whereby the portions of the ink layer have been transferred from it cannot be used again.
(3) Each portion of the ink layer, which will be one of the dots forming data to be printed on the recording paper, is not instantaneously transferred to the paper in its entirety. Rather, it is gradually transferred, from its front edge to its last edge.
FIGS. 2A and 2B illustrate a thermal ink-transfer serial printer designed, generally in accordance with the specifications described above. This printer has a carriage (not shown). As is shown in FIGS. 2A and 2B, the printer further comprises a ink ribbon cassette 110, a print head 105, and a ribbon-holding bar 109 -- all mounted on the carriage. A roll of ink ribbon 103 is set within the cassette 110. The bar 109 is not located such that portion of the ribbon 103 which extends between the head 105 and the bar 109 is kept in contact with recording paper 106 as is illustrated in FIG. 1. Rather, the bar 109 is arranged such that the ribbon 103 is already guided in a path inclined to recording paper 106 at several degrees to ten and odd degrees, when it reaches the ribbon-holding bar. Only in this respect, this printer is different from the printer shown in FIG. 1. In the case of a serial printer, it suffices to keep the ink ribbon in contact with the recording paper 106 for a short distance between the heat-generating elements, on the one hand, and the downstream edge of the print head 105, on the other hand. This is because any melted portion of the ink layer can become viscous enough to stick onto the paper 106, while moving from the heat-generating element to the down-stream edge of the head 105. Hence, the ribbon-holding bar 109 does not serve to keep said portion of the ink layer in contact with the paper 106 for the time required for this portion to become sufficiently viscous. Rather, the bar 109 functions to remove the ink ribbon 103 from the recording paper 106.
As is shown in FIG. 2A, the print head 105 is pressed toward a platen 113, whereby the recording paper 106 is held between the head 105 and the platen 113. In this condition, an electric current is supplied to the selected ones of the heat-generating elements of the head 105, whereby data is printed on the recording paper 106. During this process, the print head 105 and the cassette 110 containing the ribbon 103, both mounted on the carriage (not shown), are moved at speed V in the direction of arrow B. At the same time, the ink ribbon 103 is fed in the direction of arrow A from the supply reel 111 of the cassette 110, and is taken up around the take-up reel 112 of the cassette 110. The ribbon 103 moves relative to the print head 105 at speed -V, whereby the melted portions of the ink layer 102 are transferred from the ribbon 103 onto the recording paper 106. Since the ink ribbon 103 is pinched between the heat-generating elements, on the one side, and the recording paper 106, on the other side, the ink ribbon 103 cannot be taken up at any speed other than -V.
As soon as the print head 105 prints one line of characters on the recording paper 106, the carriage is moved toward its home position as is shown in FIG. 2B, that is, in the direction of arrow C. While the carriage is returning to the home position, the print head 105 does not press the ink ribbon 103 onto the recording paper 106, nor is the ink ribbon taken up at all.
A thermal ink-transfer printer, wherein the print head and the recording paper which move relative to each other, is regarded as unable to carry out bidirectional printing, that is, to print data not only while the carriage is moving from its home position, but also while the carriage is returning to its home position. The time required for one carriage-return is almost the same as the time required for the carriage to move from the home position to the printing-end position. Hence, unless the print head prints data also while the carriage is returning to the home position, the printing speed of the thermal ink-transfer printer cannot be increased.
It is due to the characterizing features (1) to (3) that the thermal ink-transfer printer cannot accomplish bidirectional printing. Nonetheless it is much demanded that a thermal printer which ink is thermally transferred onto the paper capable of performing bidirectional printing be developed. To make this demand realize, a variety of modified thermal printers as mentioned above have been proposed which can carry out bidirectional.
Japanese Patent Disclosures No. 61-58768 and No. 61-112682 disclose a thermal ink-transfer printer having a carriage and two head-cassette units mounted on the carriage. Each head-cassette unit comprises a print head and a cassette containing a roll of ink ribbon. The first head-cassette unit is used to print data while the carriage is moving away from its home position, and the second head-cassette unit is used to print data while the carriage is returning to its home position. Indeed this printer can achieve bidirectional printing, but it becomes large size since it has two print heads and two ink-ribbon cassettes.
Further, Japanese Patent Disclosure No. 60-224571 discloses a thermal ink-transfer printer having one print head and one ribbon cassette, both mounted on the carriage. In the cassette, an ink ribbon is set in turning back of the ink ribbon such that the two portions are able to be located one above the other near the print head and are allowed to move in the opposite directions, respectively. The print head is moved up and down relative to the two portions of the ink ribbon. The print head is pressed to the lower portion of the ink ribbon while the head is moving away from its home position, thus printing data. On the other hand, the print head is pressed to the upper portion of the ink ribbon, while the head is moving toward the home position, thereby printing data. Obviously, the printer needs an add-mechanism for move the head or the cassette up and down. Further, the cassette has a complex structure since the ink ribbon must be turned back, and the resultant two portions of the ribbon must be located, one above the other. Still further, since the ink ribbon is turned back within the cassette, it is fed from the supply reel and taken up around the take-up reel, more slowly than in the case where the ribbon travels straight within the cassette. Consequently, the printer cannot print data at high speed.
Japanese Patent Disclosure No. 60-193682 discloses a thermal ink-transfer printer which has one print head and two ink ribbon cassettes. The cassettes are located one above the other. The two rolls of ink ribbon, which are set in the cassettes, respectively, are fed in the opposite direction, respectively. This printer also needs a mechanism for moving the ribbon cassettes up and down, and is inevitably larger than otherwise.
Japanese Patent Disclosure No. 61-141582 and U.S. Pat. No. 4,577,198 disclose a thermal ink-transfer printer which uses a strip of ink ribbon having a width twice as great as the print width of the print head. The ink ribbon is moved up so that the head contacts the lower half of the ribbon, or down so that the head contacts the upper half of the ribbon. Further, the ribbon is moved in a direction or the opposite direction, in accordance with the direction in which the print head is moved to print data. Since the printer also needs a mechanism for moving the cassette up and down, it is inevitably large.
Japanese Patent Disclosure No. 61-195877 discloses a thermal ink-transfer printer which uses reusable ink ribbon and can perform bidirectional printing. The ribbon is fed from the first reel and taken up around the second reel while the print head is moving away from its home position, thereby printing data. The same ribbon is fed from the second reel and taken up around the first reel while the head is returning to its home position, thus printing data. The ink ribbon is replaced by new one when it become no longer usable. Ink ribbon reusable in thermal ink-transfer printers is generally less sensitive to heat than one-time use ink ribbon, and is hence less suitable for high-speed printing. Further, the more times the ribbon is used, the lower the print density it provides. Therefore, this thermal ink-transfer printer can hardly be recommended for printing characters or an image in various gray levels.
As has been pointed out, the thermal ink-transfer printer, which uses two ribbon cassettes or one ribbon cassette of special design to accomplish bidirectional printing, is inevitably large and expensive. On the other hand, the thermal ink-transfer printer, which uses a single cassette containing reusable ink ribbon, need not be large, but it can hardly achieve high-speed, high-quality bidirectional printing.